The production of microcellular or foamed moldings having an impervious skin by the isocyanate polyaddition process is known. It is carried out, for example, by introducing a reactive and foamable mixture based on compounds containing reactive hydrogen atoms and polyisocyanates into a mold (cf. for example German Auslegeschrift No. 11 96 864). The compounds containing reactive hydrogen atoms include polyethers containing hydroxyl groups while the polyisocyanates may be selected, for example, from 2,4- and 2,6-tolylene diisocyanate, their isomer mixtures and the polyphenyl/polymethylene polyisocyanate obtained by phosgenating aniline-formaldehyde condensates. The blowing agents used include water and/or fluorinated hydrocarbons or azodiisobutyrodinitrile. Catalysts of the type known in the production of polyurethanes are also generally used.
Assuming the starting components (it is also possible for example to use chain-extending agents, such as glycols or diamines) are suitably selected, it is possible by adopting this procedure to produce elastic products, rigid products, and any intermediate variants. The starting materials used for moldings subjected to severe stressing are generally slightly branched and, after processing, give materials having an elastomer-like property spectrum.
Processing of the starting materials to form bodywork components (e.g., automotive parts) is presently carried out by the so-called reaction injection molding process (RIM process). This process is essentially a filling process in which the highly active, liquid starting components are introduced relatively quickly into the mold through high-output high-pressure metering units after mixing in so-called positively controlled mixing heads.
DE-OS No. 26 22 951 (U.S. Pat. No. 4,218,543) describes how even extremely reactive systems, i.e. oneshot mixtures, of diisocyanates or polyisocyanates (liquid at room temperature) based on 4,4'-diisocyanatodiphenyl methane, active aromatic polyamines, relatively high molecular weight polyhydroxyl compounds containing primary hydroxyl groups and catalysts, with cream times of down to less than one second, can be processed by this method. With systems such as these, the transition from the liquid phase to the solid phase is virtually instantaneous. Although the high reactivity of systems such as these made it possible for the first time for polyurethane and polyurea moldings to be mass-produced with residence times in the mold of less than 1 minute, the known process was nevertheless confined to the production of substantially non-cellular, solid moldings because the blowing agents (fluorinated hydrocarbons or water) normally used in the production of polyurethane-based molded foams develop their blowing effect too slowly. These conventional polyurethane blowing agents proved to be unsuitable for converting highly reactive systems of the above-mentioned type into microcellular moldings having a density reduced by more than 20%. On the other hand, the use of gases, such as nitrogen or air, or of lowboiling liquids involves the problem of exact metering and the difficulty of combining such blowing agents with the reactants for the polyisocyanates to form a storable second component.
On the other hand, there is a need in practice to combine the advantage of an extremely short in-mold residence time of highly reactive systems with the advantage of the reduced weight of the moldings in order to reduce the consumption of material in the production of the moldings and to save weight and, hence, energy (for example where the moldings are used as components in automobile and aircraft construction).
One advantage of the systems described in DE-OS No. 26 22 951 lies in the fact that no external release agent must be used as a separation aid for the release of moldings from geometrically simple panel molds. However, mass production of geometrically complex parts requires the application of release agent at regular intervals. Any fine engraving of the mold gradually becomes covered by residues of release agent. The removal of these firmly adhering residues from the molds, which in many cases are heavily contoured, is only possible with considerable effort. The moldings themselves are covered by a thin film of release agent to which lacquer systems do not adhere. The moldings must be rubbed down before lacquering or washed with solvents to obtain adequate adhesion of the lacquer to the plastic.
The use of the mold release agents known from the Patent Literature which are eminently suitable for the production of self-releasing polyurethane-based foam moldings (cf. for example U.S. Pat. No. 3,726,952, GB-PS No. 1,365,215, U.S. Pat. Nos. 4,033,912, 4,024,090, 4,058,492, DE-OS Nos. 24 27 273, 24 31 968, 23 07 589, 23 19 648 or U.S. Pat. No. 4,076,695), in the reaction mixtures according to DE-OS No. 26 22 951 generally leads at best to a slight improvement in the self-releasing properties of the elastomers produced by the reaction injection molding technique. In addition, the use of release agents containing acid groups, particularly carboxyl groups, in the formulations according to DE-OS No. 26 22 951 is attended by the disadvantage that such release agents interfere with catalysis of the highly reactive systems and by the disadvantage that the moldings obtained have poor green strength.
The production of compounds containing ammonium carbamate groups by reacting primary or secondary aliphatic amines with carbon dioxide is already known. Thus, DE-OS No. 15 70 571, for example, describes the reaction of polyethers containing amino groups with carbon dioxide to form the corresponding ammonium carbamates which may be mixed with epoxide compounds, polyethers or polyesters in the production of foams. There is no reference in this publication to the production of polyurethane or polyurea foams, not to mention the production of polyurea-based moldings.
DE-OS No. 2,132,102 describes the production of polyurethane foams by reacting prepolymers containing NCO-groups with addition products of carbon dioxide and low molecular weight polyamines.
DE-OS No. 22 00 480 describes the production of cellular polyurethanes by reacting polyisocyanates with water. In this process, low molecular weight ammonium carbamates may be used in addition to the water. This publication also has no connection with the production of moldings of the type contemplated herein.
U.S. Pat. No. 3,635,908 is concerned with the production of polyurethanes from (i) polyisocyanates, polyhydroxyl compounds or NCO-prepolymers obtained therefrom and (ii) ammonium carbamates based on low molecular weight polyamines and carbon dioxide. This publication also contains no suggestion of the production of moldings comparable to the products obtained by the process of the present invention.
Accordingly, the object of the present invention is to provide a new process for the production of polyurea-based moldings which is distinguished by the following advantages:
1. the in mold residence times are limited to at most 300 seconds and preferably to at most 180 seconds in the interests of economic mass production;
2. the individual components of the system (which is preferably a two-component system) are mixtures which can be stored at room temperature and transported as such;
3. the process makes it possible to produce microcellular moldings the density of which is at least 10% and preferably between 20 and 40% lower than that of a corresponding, solid molding without any significant deterioration in mechanical properties;
4. The process makes it possible to produce moldings with an improved i.e. more homogeneous surface.
According to the invention, the object as stated above is achieved by the process described herein.